Forest microbiome and global change

Forests influence climate and mitigate global change through the storage of carbon in soils. In turn, these complex ecosystems face important challenges, including increases in carbon dioxide, warming, drought and fire, pest outbreaks and nitrogen deposition. The response of forests to these changes is largely mediated by microorganisms, especially fungi and bacteria. The effects of global change differ among boreal, temperate and tropical forests. The future of forests depends mostly on the performance and balance of fungal symbiotic guilds, saprotrophic fungi and bacteria, and fungal plant pathogens. Drought severely weakens forest resilience, as it triggers adverse processes such as pathogen outbreaks and fires that impact the microbial and forest performance for carbon storage and nutrient turnover. Nitrogen deposition also substantially affects forest microbial processes, with a pronounced effect in the temperate zone. Considering plant-microorganism interactions would help predict the future of forests and identify management strategies to increase ecosystem stability and alleviate climate change effects. In this Review, we describe the impact of global change on the forest ecosystem and its microbiome across different climatic zones. We propose potential approaches to control the adverse effects of global change on forest stability, and present future research directions to understand the changes ahead.

Stasis in forest regeneration following deer exclusion and understory gap creation: A 10-year experiment

Chronically elevated ungulate browse pressure in temperate forests worldwide often generates ecological legacies characterized by low plant diversity and contributes to the formation of dense, nearly monodominant, and highly recalcitrant layers of understory vegetation. Once established, these recalcitrant layers combined with continued browsing may jointly constrain tree establishment and diversity so completely that understory recovery may be unattainable without mitigating browse pressure, the recalcitrant layer, or both. Here, we investigate the independent and synergistic effects of both white-tailed deer (Odocoileus virginianus) browsing and hay-scented fern (Dennstaedtia punctilobula) competition on tree regeneration in a 10-year experiment. Specifically, we examine how tree seedling establishment, growth, and composition are filtered by fern cover versus fern removal (gaps), browser presence versus absence (exclosures), and their combined effects during 10 years at three hardwood forest sites in Pennsylvania, USA. Fern gaps enhanced establishment for multiple tree species, increasing seedling density and diversity, particularly in the first 3 years post-treatment, and enhancing richness (≤1 species) over the course of the experiment. Excluding deer for a decade increased the height growth of other regeneration and altered species composition, but had no effect on diversity, richness, and density. Notably, we observed higher Prunus serotina seedling densities outside exclosures, possibly due to greater secondary dispersal. We argue that browsing legacies in second growth forests established at the turn of the last century created two conditions inimical to diverse forest regeneration: an overstory dominated by two species, P. serotina and Acer rubrum (86% of basal area), and a dense recalcitrant understory layer dominated by a native fern. The first condition limits propagule supply, the second strongly filters seedling establishment, and both create impoverished forest understories composed of few individuals and species. In undisturbed forest understories, the inertia toward impoverishment was sustained across the decade even where browsing was eliminated. Consequently, stand replacing disturbances (whether natural or anthropogenic) that disrupt the understory layer and reinitiate succession may be necessary to propel forests out of their current stasis and down a pathway leading to greater diversity.

Insect Visitors of Black Cherry (Prunus serotina) (Rosales: Rosaceae) and Factors Affecting Viable Seed Production

Since 2000, wild black cherry (Prunus serotina Ehrh.) has produced erratic seed crops, especially in the Allegheny National Forest where poor seed production has been implicated in difficulties with black cherry regeneration in forest stands. Given that black cherry is insect pollinated and unable to produce viable seed from self-pollination, a reduction in seed crops could be due to a pollination deficit; however, its key pollinators are unknown. Identifying the pollinators and factors that influence pollinator abundance and fidelity is critical for supporting and enhancing seed production for this valuable timber species. Over a two-year period in developed, semideveloped, and forested areas in Pennsylvania, we identified the potential pollinators of black cherry and examined how their abundance, along with several other abiotic and biotic factors, influenced viable seed production. We found that andrenid (Andrenidae: Hymenoptera) bees are likely the most important pollinators. The proportion of viable seeds increased as the number of andrenids increased, and these ground nesting bees were most abundant on forest edges, highlighting this habitat's potential to support pollination services. Andrenids carried an average of 347-fold more black cherry pollen than flies and 18-fold more than halictid (Halictidae: Hymenoptera) bees. We did not find a significant relationship between the abundance of any other taxa besides andrenids and viable seed production. Black cherry flowers also provide resources for natural enemies such as the economically important parasitoid of Popillia japonica Newman (Scarabaeidae: Coleoptera), Tiphia vernalis Rohwer (Tiphiidae: Hymenoptera), which was observed feeding on black cherry nectar in this study.

Long-term monitoring of Vermont's forest soils: early trends and efforts to address innate variability

Long-term monitoring of forest soils is necessary to understand the effects of continued environmental change, including climate change, atmospheric deposition of metals, and, in many regions, recovery from acidic precipitation. A monitoring program was initiated in 2002 at five protected forest sites, primarily Spodosol soils, in Vermont, northeastern USA. Every 5 years, ten soil pits were sampled from random subplots in a 50 × 50-m plot at each site. Samples were taken by genetic horizon and, to reduce variability and improve comparability, from four specific layers: the combined Oi/Oe layer, the combined Oa/A layer, the top 10 cm of the B horizon, and 60-70 cm below the soil surface (usually the C horizon). The samples were archived and a subset analyzed for carbon, nitrogen, and exchangeable cations. After four sampling campaigns, the average coefficients of variation (CVs) at each site had a broad range, 10.7% for carbon in the Oa/A horizon to 84.3% for exchangeable Ca²⁺ in the B horizon. An investigation of variability within the upper 10 cm of the B horizon across a 90-cm soil pit face showed similar CVs to the entire site, emphasizing the need for consistent and careful sampling. After 15 years, temporal trends were significant in the Oa/A and B horizons at two of the five sites, with one site showing an increase in carbon concentration in both layers along with increases in both exchangeable Ca²⁺ and Al³⁺ in the B horizon, perhaps linked to recovery from acidification. The monitoring program plans to continue at 5-year intervals for the next century.